Electrical power companies and the customers of power companies have a mutual interest in reducing the amount of power wasted in a power delivery system. Power companies engineer transmission lines, transformers, and generators to provide the power that all of their customers will draw. Even though some of the power drawn by the customers is not used by the customers, the power companies still have to engineer transmission lines, transformers, and generators to provide this additional wasted power. Further, some power transmitted to customers is not used by the customers or wasted, but is “reflected” back to a power generator. Thus, the transmission lines have to carry both the transmitted power and the reflected power. Not only does this mean that the transmission lines must be engineered to carry both the transmitted and reflected power, but it also means that losses in the transmission lines, transformers, and loads are increased because there are losses both in the transmitted power and the reflected power traveling through the power system.
The power factor is the ratio of the real power supplied to a customer compared to the sum of the power supplied to the customer and the power reflected back to the power company is known as the power factor. A power factor of 1 is considered ideal. Power companies typically charge residential customers only for real power. Industrial customers, however, may be charged for real power with an additional charge for power factor. Usually, a power company may not apply an additional charge for power factors above a threshold, but industrial customers may be charged in proportion to the power factor below that threshold. The threshold varies for each power company, but is generally between 0.85 and 0.95. Thus, if the power company sets the threshold at 0.95, and the customer's power factor is 0.85, then the power company may charge a fixed tariff on all of the real power used. Typical tariffs for poor power factor can be about 10%.
Correcting power factor has benefits other than the cost of the power. The internal electrical capacity of the customer system increases because lower currents are required to deliver the same power. As a result, additional equipment can be powered without providing increased capacity wiring, switch boxes, and transformers. Voltage drops at the point of use may be reduced, and under-voltage reduces the load that motors can carry without overheating or stalling.
Reduced power factor can be caused by several mechanisms. A first mechanism is reactive loads caused by capacitors, inductors, or some combination of capacitors and inductors. These loads shift the phase of the current supplied to the customer relative to the voltage. The phase shift means that during some parts of the alternating current (AC) cycle, excess power is delivered to the customer in addition to real power consumed, and at other parts of the AC cycle, the excess power is returned to the power company. The power factor reduction by this mechanism can be corrected by adding a suitable cancelling inductor or capacitor to the customers power circuit. One issue with adding the cancelling inductor or capacitor is that the required inductor or capacitor may vary depending on how the equipment of the customer is used. Some systems adapt to changing use by switching in or out additional capacitors or inductors.
Nikola Tesla introduced induction motors. Induction motors present a lagging power factor to the power line dependent on the load. A large loaded induction motor can have a power factor as high as 0.90. The power factor for a small low speed motor can be as low as 0.5. An induction motor during a startup can have a power factor in the range of 0.10 to 0.25, rising as the rotor spins faster.
As a second reduced power factor mechanism, a customer could not take as much power from all parts of the AC cycle. Switch mode power supplies, for example, take most power at the peak of the voltage cycle. This tends to “flatten” the shape of the sine wave of the power signal, causing harmonics. The harmonics generate unwanted signals on the power line that are reflected back toward the power company as well as other customers. The harmonics are, thus, wasted power, as far as the customer is concerned. The unwanted harmonics can be removed using filters.
A third reduced power factor mechanism is energy in the form of spikes and harmonics generated out side of the customer's premises that are transmitted to the customer. Although, the spikes and harmonics travel through the power meter these spikes and harmonics cannot be usefully used by the customer, and may harm equipment.